One challenge facing the world is the harvesting, production, and distribution of energy to support economic prosperity with responsible environmental stewardship. For example, in 2010, energy consumption in the U.S. was at a rate of 3 TW, of which 83% originated from fossil fuels. Thus, finding reliable and robust alternatives to petroleum-based fuels would enhance U.S. energy independence and energy security.
One way to mitigate the risks of relying on liquid fossil fuels is to develop alternative sources of energy, such as biofuels or synthetic fuels. One approach is the use of biofuels (i.e., biofuel mixed with ordinary petroleum-based fuel). However, the main drawback with the use of these fuels is the cost; biofuels can cost about 10 times as much as petroleum-based fuels. Currently, the synthesis of these new biofuels often involves the hydrogenation of animal or vegetable fats as a step in the production of liquid fuel. Long-term, however, a drawback to the widespread use of such biofuels is the cost to produce the hydrogen involved in the hydrogenation process. Hydrogen is typically produced through high-temperature steam reforming of hydrocarbons such as methane or liquid fuels. This process is energy intensive and also relies on natural gas, which is a fossil fuel, and thus the production of hydrogenated fuels will suffer to a great extent from the same risks as petroleum based fuels. Furthermore, a recent study concluded that the complete life-cycle process of biofuels synthesis actually increases greenhouse gas emissions.
The successful utilization of the clean energy carrier hydrogen (H2) in the synthesis of biofuels, synthetic fuels (i.e., from syngas), or even as a fuel itself, requires methods for H2 production using primary energy sources not based on fossil fuels. Of possible primary energy sources, solar offers the greatest long-term impact because of its abundance and availability, but many challenges need to be met for its utilization. For the direct conversion of sunlight to stored chemical energy in H2, efficient photochemical reduction of protons in water is needed.
Molecular hydrogen can be produced from protons (H+) in the reductive half-reaction of artificial photosynthesis (AP) systems. One of the strategies for light-driven proton reduction features a multicomponent solution with a light-absorbing molecule (chromophore) that transfers electrons to a catalyst that reduces protons. However, these solution systems often use nonaqueous solvents, and always have short lifetimes from decomposition of the chromophore over a period of hours. This difficulty has led to more complicated architectures that separate the sites of light absorption and proton reduction. Heterostructures between nanocrystals (NCs) and traditional precious metal nanoparticle H2 production catalysts, and between NCs and iron-hydrogenases, have produced proton reduction in solution.
Research in artificial photosynthesis has been active since the 1970's. The energy storing reaction that is of greatest importance in artificial photosynthesis is the decomposition of water into its constituent elements, H2 and O2, with the former as the fuel. As a redox reaction, water splitting can be divided into its two half-cell components for separate investigation and development. Despite great efforts over the past decade, neither half-reaction has been carried out photochemically in a system composed of earth-abundant elements with both an activity and robustness of the type needed for further development. One approach for solar energy conversion relies on systems designed and built entirely on the molecular level, either to use light to achieve chemical potential in the form of charge separation, or as systems for solar fuel generation. Entirely molecular systems can present synthetic challenges, and molecular chromophores are prone to photodegradation. Reduced photosensitizers are unstable and rapidly photobleach, thus limiting the amount of H2 potentially produced. Organic photosensitizers can store and therefore deliver only one electron at a time, while two are required for H2 production. Thus, the turnover frequency for the overall catalytic processes is relatively slow, being dependent on diffusion of two photoexcited sensitizers to the catalyst. This second problem can be somewhat mitigated with a higher photon flux; however, a higher excitation rate exacerbates the photobleaching problem. Homogeneous systems for light-driven reduction of protons to H2 typically suffer from short lifetimes because of decomposition of the light-absorbing molecule and/or catalyst, if present.